Influence of the dorsal and median raphe nuclei on neurons in the periaqueductal gray matter: Role of 5-hydroxytryptamine

Abstract

In rats anaesthetized with urethane, selective activation of neuronal perikarya in the dorsal raphe nucleus evoked inhibitory ( n = 17) and excitatory ( n = 10) responses in single neurons recorded in the dorsolateral and lateral sectors of the periaqueductal gray matter and in the adjacent tegmental area. A further 11 cells showed biphasic inhibitory/excitatory responses. Ongoing activity of > 85% of the cells was inhibited by iontophoretic application of 5-hydroxytryptamine (1–70 nA). The duration of the inhibitory response evoked from the dorsal raphe nucleus was increased by 66–725% during iontophoretic application (3–10 nA) of the 5-hydroxytryptamine (serotonin) reuptake blocker paroxetine (five of six cells). In contrast, excitatory responses evoked from the dorsal raphe nucleus were either reduced ( n = 3) or replaced by inhibitory responses ( n = 2) in the presence of paroxetine. Paroxetine also produced a reduction in baseline firing and potentiated the responses of neurons to iontophoretically applied 5-hydroxytryptamine. Stimulation in the median raphe nucleus did not produce any significant changes in the activity of five neurons tested in the periaqueductal gray matter. It is suggested that the inhibitory influence of the dorsal raphe nucleus on cells in the dorsal half of the periaqueductal gray matter is mediated by 5-hydroxytryptamine. This projection may be involved in modulating the level of excitability of neurons in the midbrain aversive system which integrate defence behaviour. In addition, there appears to be a non-serotonergic excitatory projection from the dorsal raphe nucleus to the periaqueductal gray matter. The functional role of this projection remains obscure. The median raphe nucleus does not appear to have a significant influence on the excitability of neurons in the dorsal half of the periaqueductal gray matter.